Method For Processing A Biomass Containing Lignocellulose

ABSTRACT

There is disclosed a method for processing a biomass (for example straw) containing lignocellulose such that cellulose and hemicellulose are made accessible for further processing, typically by decomposition, without needing energy-consuming dissolution of the biomass in water. The method includes repeated compressions of the biomass in a reciprocating piston press, where loose biomass is continuously fed into a piston chamber in front of a piston which moves the loose biomass into a tubular reaction chamber in which the biomass is compressed for producing a vapour explosion and autohydrolysis under simultaneous displacement of compressed biomass through the reaction chamber. After compression, to the biomass can be added fluid livestock manure, fluid waste water sludge etc. in a biogas plant for subsequent biogas processing.

FIELD OF THE INVENTION

The present invention concerns a method for processing a biomass (forexample straw) containing lignocellulose such that cellulose andhemicellulose are made accessible for further processing, typically bydecomposition, which preferably is an enzymatic decomposition. Morespecifically, the invention concerns a method wherein processedlignocellulose is used for producing biofuels such as for exampleethanol, butanol, hydrogen, methanol and biogas.

The invention has appeared in connection with treatment of straw. Atseveral points the invention is therefore explained with reference tostraw, but by these explanations it is understood that correspondingadvantages are achieved by other kinds of biomass that containlignocellulose.

BACKGROUND OF THE INVENTION

First generation bioethanol is mainly produced on the basis of cerealcrops like wheat and maize as well as sugar cane. This is due to thefact that corn and sugar cane contain readily accessible carbohydratessuch as starch that can be converted into sugar in a simple way, andwhich is subsequently is fermented into ethanol.

However, this production has been critisised for converting goodfoodstuffs into energy apart from not being sustainable. For some years,research has therefore been made into utilising crop residue from foodproduction for production of biofuel, in particular bioethanol. Researchhas particularly concentrated on converting straw and wood chips intobioethanol. This type of ethanol is labelled second generationbioethanol or cellulosic ethanol.

Biomass, such as wheat straw and straw from other corn and maize cropsand wood, consists largely of cellulose, hemicellulose and lignin why itis also collectively called lignocellulose.

Cellulose is a linear homogenous polymer of up to 15,000 glucose unitsinterconnected by β-1,4-glucoside bonds. Hemicellulose is, however, aheterogeneous branched polymer with a length up to 200 units which canconsist of e.g. arabinose, xylose, galactose, mannose and glucose.

Lignin constitutes a network formed by polymerisation of the monomersp-coumaryl alcohol, coniferyl alcohol and sinapyl alcohol. The complexnetwork of lignin encapsulates and contributes to binding cellulose andhemicellulose together. The structure of the plant cell wall is herebystrengthened and protected against decomposition in the nature e.g. byattacks from fungi or insects. In general, lignocellulose contains about35-50% cellulose, 20-30% hemicellulose and 15-30% lignin.

However, there are great differences in the contents of various plantsand the composition of hemicellulose and lignin is very dependent on thespecies. In general, wood contains more lignin and less hemicellulosethan straw, and where hemicellulose in straw mainly consists ofarabinose and xylose, in conifers it contains mostly mannose and only alittle xylose.

Utilisation of lignocellulose as substrate for various fermentationprocesses presupposes a prior decomposition of cellulose andhemicellulose into their respective monomers. The first step in thisprocess is a thermochemical treatment of the lignocellulose wherebylignin is released and hemicellulose and cellulose are partiallydissolved or made more accessible to enzymes.

The enzymes for decomposing lignocellulose can be divided into two maingroups: cellulases and hemicellulases. The last step in thedecomposition of cellulose is the cleavage of cellobiosis into twoglucose molecules by the enzyme β-glucosidase. The more heterogeneousstructure of hemicelluloses means that a greater number of differentenzymes are required to completely decompose it into sugar molecules. Anexample of such a complex enzyme mixture is Novozyme's Cellic.CTec3which contains different cellulases and hemicellulases as well as otherhydrolytic enzymes.

As mentioned, the first step in the utilisation of lignocellulose is apre-treatment and typically a thermochemical pre-treatment. Steamexplosion is one of a wide range of such different thermochemicalpre-treatment methods. This process is combined with addition of waterand catalysts such as acids and bases or gases like oxygen and sulphurdioxide.

Pre-treatment of biomasses like straw and wood chips for making fluidbio-fuels, especially ethanol, has been subject to a very comprehensiveresearch effort, and a massive amount of scientific literature is thusavailable on this area.

In the recent years dominating biochemical methods have been described.A comprehensive presentation of these works is not to be made here, butit is, however, to be noted that several groups point to autohydrolysisas the preferred technology because it is not based on chemicals,because formation of inhibitors is modest and because biomass with arelatively high dry matter content can be processed. It is alsopreferred by most authors over wet oxidation in which oxygen is added tothe process.

Autohydrolysis is termed differently but is often called thermalhydrolysis, steaming or steam explosion regardless that the explosionpart is not necessarily an advantage to the hydrolysis or comminution ofthe material. The method borders to “liquid hot water treatment”,depending on the amount of water, and wet oxidation if oxygen forms partof the process.

The scientific literature furthermore points to the use of a number ofchemicals and catalysts or to hydrolysis of lignocellulose, includingweak and strong acids and bases and a number of gases like SO₂, CO₂, O₂,NH₃, H₂O₂, O₃. To this is added application of enzymes, either madeindustrially or as a biological pre-treatment.

The technical installations used for such thermochemical pretreatmentsof lignocellulose-containing biomasses have only been made in a fewexamples.

The best known apparatus is the staketech hydrolysator of SunOpta whichis used in the first commercial plant for producing bioethanol based onstraw. This machine has a horizontal reaction chamber with a screwconveyor moving the straw forward under high pressure and temperature,allowing it to explode into an associated expansion container atfrequent intervals, i.e. at intervals of a few seconds. The operatingtemperature and pressure are 190-210° C. and 15-20 bars, respectively.

The Atlas Stord hydrolysator for hydrolysis of feathers uses a differentprinciple, so-called plug flow, where the reaction chamber is a verticalchamber with a valve at the bottom which opens and closes at intervalsof a few seconds. The overpressure in the reaction chamber will thusmake the hydrolysed feathers to explode into an expansion container. Thereaction chamber is therefore not provided with a shaft passage. Theoperating temperature and pressure are 160-210° C. and 6-10 bars,respectively.

Finally, Villavicencio (1987) has published an invention forthermochemical treatment of fibres by means of several reactionchambers. The biomass is supplied via screw conveyors, which also act asback pressure valves, to the first reaction chamber.

Common to all techniques are that

-   -   1) heat is supplied from an external heat source, particularly        by means of hot water or steam;    -   2) water in the form of liquid water or steam is added to the        process such that the dry matter content is at most 30-40% in        the reaction chamber, and typically 10%;    -   3) water or steam is added as a necessary prerequisite for        treatment at high temperatures at the level of 160-220° C.

The operational mode of the technique, and as the name “steam explosion”indicates, is a mechanical decomposition of the fibres of the biomass bya steam explosion caused by a sudden pressure drop from e.g. 20 bars toatmospheric pressure. The state of water at e.g. 200° C. under pressureis as a liquid, but when the pressure abruptly drops to atmosphericpressure, part of the water is transformed into steam, meaning the wateroccurring in all parts of the plant fibres as well. When this waterexplodes in the cellulose fibres, the biomass is torn up mechanically.This tearing up contributes to make the component parts oflignocellulose of cellulose and hemicellulose accessible for furtherprocessing, as for example by enzymatic decomposition.

Conventional steam explosion is often accomplished at temperatures inthe range 160-220° C. and corresponding pressures at 0.60-4.83 MPa. Theprocessing time varies from a few seconds to several minutes before thematerial is exposed to atmospheric pressure via explosive decompression.The process causes decomposition of hemicellulose and transformation oflignin due to the high temperature. Hemicellulose is decomposed byacetic acid and other organic acids formed during the treatment, i.e.via so-called autohydrolysis. Lignin is not decomposed to the samedegree but is redistributed on the fibre surfaces as a result of meltingand depolymerisation/repolymerisation reactions.

Besides these chemical effects, steam explosion also has a purelymechanical or physical effect as the material explodes and fragmentswhereby the accessible surface is increased.

The procedure is implemented, as mentioned, by adding water to thebiomass, either in the form of liquid water or in the form of steam, ora combination thereof, and heating the mixture. High temperatures areattained by heating with hot water or steam.

The highest thy matter concentration achieved by these systems is about30-40%, typically much lower, requiring large technical installationsdue to the amount of water and the voluminous structure of the biomass.Even a compressed straw bale has a density of about 150 kg/m³ which isnot much.

A crucial challenge to the technique is the large amounts of water andenergy used for pre-treatment and the necessarily large installationsfor pressure containers, valves, pipes, screw conveyors etc.

This also entails substantial drawbacks by biogas plants since the largeaddition of water with the straw strains the hydraulic capacity of abiogas plant, and since the energy consumption reduces the net energyproduction and the cost efficiency.

A biofuel can also be provided in the form of biogas. Until now biomass,preferably in the form of straw, has not been used for biogasproduction. It is not known to use straw for biogas purposes. It is onlyknown that straw forms part of biogas plants to the extent that straw isused as bedding in livestock production and to the extent that theresulting livestock manure is degassed.

Actually, it is rather surprising that straw is not used for biogaspurposes. In the light of the fact that livestock manure, i.e.essentially cattle and pig liquid manure, is fluid with a dry mattercontent between 4 and 8%, there is room for additional dry matter in thebiogas plant, in particular straw.

Straw is a difficult material to handle. It is very abrasive, veryhydrophobic and has a very low density, i.e. less than 100 kg per m³.The handling of straw in any connection and in particular in biogasplants therefore requires a special technique.

In addition, straw predominantly consists of cellulosic fibres which arecrystalline polymers of (1-4)-β-D-glucose. Hemicellulose forms partthereof which correspondingly is an amorphous and partly crystallinepolymer consisting of (1-4)-β-xylose. Hemicellulose forms part of bothfibres and cell walls. Lignin, a third essential component of straw, isa polymer of phenol. Hemicellulose as well as lignin protect thecellulose against “weather and wind”, and in this connection againstdecomposition by enzymes and microorganisms.

In order to efficiently utilise straw in a biogas plant it is thusnecessary to pretreat the straw in order to open up the fibres of thestraw and to make the component parts of the lignocellulose accessibleto decomposition. As mentioned above, this will be energy-consuming andnecessitate use of voluminous plants.

Object of the Invention

The object of the present invention is to indicate a method forprocessing a biomass (for example straw) containing lignocellulose suchthat cellulose and hemicellulose are made accessible for enzymaticdecomposition, in particular with the intention of making biofuels suchas for example ethanol and biogas.

Description of the Invention

According to the present invention, this is achieved by a method whichis peculiar by including steps for:

-   -   repeated compressions of the biomass in a reciprocating piston        press, where loose biomass is continuously fed into a piston        chamber in front of a piston which moves the loose biomass into        a tubular reaction chamber in which the biomass is compressed        for producing a mechanically induced water vapour explosion and        autohydrolysis under simultaneous displacement of compressed        biomass through the reaction chamber.

By the present invention is thus achieved an efficient method forestablishing a first step in the process of utilising lignocellulose assubstrate for various processes as the explosion of the water by themechanically induced steam explosion causes the cellulose fibres to betorn up mechanically. This tearing up make lignocellulose components ofcellulose and hemicellulose accessible to subsequent enzymaticdecomposition to their respective monomers.

A continuous feeding of the biomass and a simultaneous displacing ofcompressed biomass through and out of the reaction chamber enable acontinuous process in a plant in which there is only need for aprocessing unit with a very restricted volume. A piston press withcapacity of processing 1 ton of biomass per hour can thus have a sizeless than 3 cubic metres. A further development for larger machines mayfurther optimise this ratio.

The piston stroke acts on the biomass with a pressure between 500 and3000 bars, in particular between 1000 and 2500 bars. The biomass ishereby compressed to 500-1000 kg/m³ and is directly impactedmechanically. At the same time, the kinetic energy of the piston isdeposited in the straw in the form of heat.

The heat formation in the biomass primarily occurs because of frictionbetween the biomass and the walls of the reaction chamber and internalfriction in the biomass. The heat formation causes a strong heating ofthe walls of the reaction chamber and a lesser heating of the biomass.The walls are typically heated to between 110 and 200° C., the biomassto between 60 and 170° C., though locally the temperature rises above200° C. The compression in the reaction chamber causes the occurrence ofvery many local steam explosions.

As this water is under pressure, it remains in liquid state until thepiston is retracted before a new piston stroke. At retraction, the waterexplodes and the biomass is impacted as by a steam explosion. This isrepeated a number of times until compressed biomass is advanced so farin the compression chamber that piston strokes do no longer influencethis biomass.

The action of heat and the steam explosion cause a certainautohydrolysis of the biomass, meaning that steam at high temperaturepartly dissolves the lignocellulose by a hydrolytic process. By theautohydrolysis there are generated organic acids which lower pH to 4-6,typically pH 5.

The process is distinguished by being very energy economic as there isno need for heating large amounts of water.

Summarising, it can be said that steam explosion is a technique withseveral cooperating effects: effect of high temperature (i.e. formationof organic acids, lignin melts); effect of autohydrolysis (hemicelluloseand partly lignin are decomposed via activity of i.a. acetic acid); andeffect of mechanical tearing up.

A mechanical press is designed as an eccentric press. Mechanical pressesinclude a constantly rotating drive mechanism converting a rotatingmovement into a reciprocating movement of a piston by means of aneccentric. The piston has two extreme positions. At one position thepress face of the piston is located in a piston chamber, also calledprecompression chamber, with the material to be pre-treated, inparticular by compression into a briquette, and at the other extremeposition the press face of the piston is located at the inlet to an openconical nozzle at the side of the precompression chamber. On its wayfrom one extreme position to the other, the piston pushes some of thematerial from the chamber in front of itself into the nozzle. Thematerial portions compressed and pre-treated by each stroke of thepiston, or in concrete situations formed bio-briquettes, arecontinuously pushed out through the outlet of the nozzle. Mechanicalpresses operate at far higher pressures than hydraulic presses as apressure of at least 800 bar is attained. In a bio-briquette made in ahydraulic press the bonding of the biological material is primarilymechanical and secondarily by adhesion, whereas the bonding of thebiological material in a bio-briquette made in a mechanical press isprimarily by adhesion and secondarily mechanical. The present inventionis used within the technical area of mechanical briquette pressingmachines as it concerns high capacity production of bio-briquettes orpre-treatment of biological material.

Reciprocating mechanical briquette-pressing machines for makingbriquettes, mainly briquettes of wood or other usable biologicalmaterials such as fabric, MDF dust, plant fibres, straw, hemp, bark,paper, cardboard, coal dust, domestic waste, livestock manure or sludge,are known. The briquettes can primarily be used for firing in solid fuelfurnaces for e.g. domestic space heating. The material is typically aresidual product from the wood industry in the form of sawdust orshavings.

The material is to have a moisture content of 5% to 20%, typically 6% to16%. We are here speaking of percentage by weight. The material iscompressed in the die under great pressure and consequent hightemperature. The biological material contains cells that among othersinclude water, cellulose and lignin. The purpose of the compressions isto activate the lignin which after cooling provides for binding thematerial (the bio-briquette) together. During application aspre-treatment and possible addition of organic acid, this is the basefor extracting lignin and thereby exposing cellulose and hemicellulosefibres to further processing. The rising pressure in the biologicalmaterial produces a rise in temperature in the cells, causing the waterin the cell to be transformed into steam by a steam explosion wherebythe cell wall is destroyed and the lignin is released. The steamexplosions are initiated at a pressure of about 400-500 bar and continuewhile the pressure rises to the maximum value of more than 2000 bar. Ifthe moisture drops below 6%, there is normally not enough moisture inthe material for producing enough steam explosions so that a bonding cantake place. If the moisture rises over 16%, the steam explosions usuallybecome so strong that the process fragments the briquettes, and thelatter are flung out of the machine or back into the system. This can beadvantageous as pre-treatment as pre-treatment as such is desired ratherthan formation of an actual briquette.

As it appears from the above, a more complete decomposition of the cellsunder formation of the briquette in a mechanical briquette press isachieved due to the higher pressure. The amount of lignin released forsubsequent bonding of the bio-briquette is substantially higher.

The biological material leaves the briquette press as a continuous rod.Each piston stroke adds, so to say, a “disc” of biomass to the run ofmaterial, and surfaces of fracture are formed between each disc.Mechanical presses are typically used in larger installations from about200 kg/hour and up to about 2500 kg/hour. In a mechanical press thedesired back pressure can therefore only be adjusted by mounting anozzle with a different conicity or with a variable squeeze nozzle. Dueto the fact that the mechanical press is driven by electric motors andnot by a hydraulic motor there is only a small energy loss in themachine, and the ratio between production and power consumption istherefore optimal. The service life cycle of a mechanical press isconsiderably longer than that of a hydraulic press.

It is possible to perform the invention as a decentralised solution,meaning that compression for formation of briquettes is performed at onelocation and that the briquettes are stored and later transported to afacility for decomposition, as for example a biogas plant or abioethanol plant.

By the invention it becomes possible to compress biomass to highdensity, to supply heat via mechanical kinetic energy, to avoid additionof water, and to use the natural water content of about 5-20% andtypically 6-16% of a biomass for repeated steam explosions. The processhereby becomes rational in that it is exclusively the biomass which istreated at high temperatures—and not a large amount of water—and thatthis occurs in very small reaction chambers.

Compression of wood and straw is known from pressing these materialsinto briquettes or pellets for subsequent combustion. However, it is notknown to optimise the mechanical compression for application asmechanically induced steam explosion of biomass such that cellulose andhemicellulose are made accessible for enzymatic decomposition beforefermentation into ethanol or other biofuel.

By the present invention is achieved a very high specific density of thestraw between 800 and 1200 kg/m³, typically a bulk density between 500and 600 kg/m³, considerably reducing the size of the reaction chamber(due to high specific density) and the need for possible transport to acentral processing plant (due to high bulk density). Among the specialadvantages achieved by the present invention is thus a compact reactionchamber. Only a few litres of reactor volume is used, i.e. less than 50litres and typically about 10 litre, as opposed to frequently severalcubic metres in other systems (5-10 m³ or more).

Addition of water is avoided and the biomass, e.g. in the form of straw,is therefore treated at its natural water content of 5-20%, typicalbetween 6 and 16%. This substantially reduces the energy requirement asthe heat capacity of water is about 4.2 J/gK whereas the heat capacityof dry straw and wood is about 1.2 J/gK. A typical addition of water 10times the weight of straw by thermochemical pre-treatment thereforeincreases the energy consumption with about 40 times in the directprocess.

If expedient in a given process, lignin can be extracted aftermechanical steam explosion, but then at temperatures below 100° C. andtypically around 50-80° C. Lignin can be extracted by water only or byacids or bases according to known prescriptions for extraction oflignin. Here, typically organic acids as lactic acid, citric acid oracetic acid are applied which possibly can be added before pressing andcontribute to hydrolysis as well as extraction of lignin.

The straw is impacted with greater mechanical intensity as the straw isimpacted directly by the piston strokes under compression and byrepeated steam explosions as well. This provides a far betteraccessibility for enzymes during the subsequent enzymatic reaction suchas liquefaction and saccharification before ethanol fermentation andthereby a lesser need for enzyme addition.

There exist a number of commercial enzymes for liquefaction andsaccharification of cellulose/hemicellulose. It is estimated thatconsumption can be reduced to below 50% and typically to 20% of normalconsumption by conventional thermochemically processed straw.

The heat treatment of the biomass is adapted such that it runs attemperatures within a range from 40° C. to 240° C., preferably withmeasurable temperatures typically from 60° C. to 170° C., andparticularly in the range between 60° C. and 120° C. The processing timecan be adjusted between 1 and 30 min and particularly between 1 and 5min. As only the straw is heated and processed in a compact reactorchamber there are no practical limits to the heat treatment as afunction of temperature and time. The treatment can be optimised withoutbeing limited by such considerations. When needing longer time for heattreatment, including hydrolysis, the nozzle is extended into a pipe orinsulated screw conveyor which allows for a retention time of 1-2 hoursor more. Typically, there can be a need for supplementary heat treatmentand hydrolysis for an hour at 90° C.

The straw achieves an increased waterabsorbing capability. It appearsthat straw can absorb between 2 and 15 times its own weight in water andtypically between 5 and 10 times its own weight.

The straw becomes directly mixable with water and enzymes. The additionof surfactants is normally not required in order to enhance mixing withwater and the action of enzymes.

A significant dissolution of lignins is achieved due to heat andpresence of oxygen during the process. The partial pressure of oxygen inwater is about 2×10⁻⁵ atmospheres (1 atm=101.325 kPa); the partialpressure of oxygen in the atmosphere is about 2×10⁻¹; the partialpressure is thus 10⁴ times greater in the atmosphere than inoxygen-saturated water. Oxygen is therefore added by wet oxidation underpressure, i.e. 5-20 atm, typically 10 atm, but still there is limitedaccess to the reaction of oxygen with lignin due to the addition oflarge amounts of water to the process. During mechanical steam explosionstraw and the ambient atmosphere with about 20% oxygen are subjected toa pressure of the said max. 2000-2500 bars. The oxygen is therefore muchmore reactive than during conventional wet oxidation, and lignin istherefore destroyed to a greater extent.

Besides, the straw can be impregnated with gases and/or bases or acids,cf. the above mentioned thermochemical methods for pre-treatment oflignocellulose before introduction to the piston chamber. This can takeplace in a mixer or a free-fall mixer.

Finally, enzymes and water can be added after treatment by means of anozzle which sprays the mixture across the dry straw in a free-fallmixer. Enzymes and water will hereby be distributed evenly across thestraw, and the particular new waterabsorbing ability of the latter willin particular distribute moisture and enzymes to all parts of the straw.

The moistened straw with enzymes can now be liquefied (hydrolysed) andsupplied to an enzyme-membrane reactor where cellulose and hemicelluloseare finally saccharified into sugar oligo- and monomers. In the reactor,the associated membranes will retain lignin and other unconvertedsubstances while the sugars pass on to ethanol fermentation. In othersetups it may be an advantage to ferment the total mixture of lignin,sugar etc.—a so-called “whole slurry”—and to separate after fermentationand distillation. This particularly depends on the amount of lignin inthe biomass.

The new technical means for use in steam explosion of biomass,preferably straw, includes a piston. This will be mounted on a crank forestablishing the reciprocating movement which moves loose biomass strawfrom a piston chamber into a reaction chamber. The latter is preferablyformed of an open pipe with a funnel-shaped nozzle in which the biomassis compressed at a pressure between 500 and 3000 bar, in particularbetween 1000 and 1500 bar.

Back pressure is established by means of the biomass (the straw) whichis accumulated and compressed in the reaction chamber and which is movedthrough the compression chamber in compressed form and by the frictionbetween the biomass and the wall of the chamber.

The length of the reaction chamber and insulation thereof are adaptedaccording to need depending on the duration of the action oftemperature. The chamber is provided with heat jacket such that thetemperature can be adjusted according to need.

It is preferred that the biomass (the straw) can be cut to a fewcentimetres of straw length. Also, it is preferred that a cleaning ofthe biomass of stones and sand and other foreign bodies is performedbefore compression.

The press is provided with thermometers and manometers according toneed.

The temperature in straw is regulated by means of the stroke force ofthe piston, cooling of reaction chamber and insulation of reactionchamber.

The finished, compressed straw can be crumbled afterwards, againappearing like cut straw though much softer now. The straw has, however,completely changed its character after the treatment and has becomewaterabsorbing, among others. The straw can absorb between 2 and 15times its own weight, in particular 5-10 times its own weight.

Hereby it is possible to add enzymes and water simultaneously, e.g. viaspray nozzles, in a free-fall mixer or other kind of mixer. Water andenzymes are thereby evenly distributed in the straw.

It is also possible to add the straw in compressed form and directly toa bioreactor, thermoreactor, chemical reactor, thermochemical reactor orother kind of reactor. Besides, it is possible to add the straw to fluidlivestock manure, fluid waste water sludge etc. before a biogas processwherein the straw then will be converted in the biogas reactors intobiogas with maximum yield.

A method according to the invention can be used in pre-treatment ofstraw for use in biogas production. A typical biogas plant degassing100,000 tons of fluid livestock manure and delivering the gas to adecentralised combined heat and power plant may—with propertechnique—without substantial further investments in the biogas plantitself utilise e.g. 10,000 tons of straw yearly as well. The biogasproduction will hereby be increased from about 2.5 m m³ from livestockmanure with about 4 m m³ from the straw to 6.5 m m³ in total yearly. Themethod provides possibility of a substantial increase in the biogasproduction in existing plants.

The total effect of mechanical steam explosion includes mechanicalcompression, heat treatment, steam explosion, oxidation andautohydrolysis.

The method according to the invention can, for example, be performed inthe following way which is described on the basis of straw but which canbe used analogously on other lignocellulose-containing biomasses.

Prior to mechanical steam explosion, the process starts by feeding drycut straw, dry sawdust or similar lignocellulose into a piston chamber.A piston on a crank moves loose straw into a tubular reaction chamber.The piston moves back and forth by the crank and moves new straw intothe reaction chamber by each stroke. Compressed straw is pushed throughthe pipe by renewed feeding of straw and compression thereof.

The straw can be impregnated by gases, acids or bases according to needbefore being introduced to the piston chamber. The autohydrolysis can beenhanced hereby and pH be lowered further in the treated material, i.e.to pH 1-4, typically pH 2. Alternatively, base can be added and thus abasic hydrolysis is performed in addition to the mechanically inducedeffects.

The compressed straw can be crumbled subsequently and is thus open toaddition of water and enzymes in a free-fall mixer or other kind ofmixer.

The treated straw is supplied to a biogas process, a bioethanol processor other fermentation process or process for producing biofuel, organicacids or other organic biological products like paper, industrialchemicals, fodder, or other material.

Commercially available mechanical components can be utilised for theinvention, including lines for handling straw in the form of big bales,including conveyor belts, tearing up, comminution to a desired particlesize by means of hammer mills, separation of stones, sand and othercontaminants before mechanical steam explosion.

Commercial briquette presses can thus be used as well after modificationas to provide the process parameters which are necessary for inducingsteam explosion in straw and similar biomass.

After the mechanical steam explosion, the material can fit intoproduction of bioethanol, biogas or other form of biofuel; typically itwill be bioethanol. In the production of bioethanol there are inprinciple two systems which either use the material directly in theethanol process or which uses an extraction of lignin before the ethanolprocess.

Finally, the fact that the treatment results in compression of thematerial to high density can be utilised in several ways. Firstly, asubsequent treatment in thermochemical or other reactor can occur athigh density as the straw is compressed and anyway can be introduced inreactor (as e.g. a bioreactor). However, it can be utilised as well thatthe biomass, e.g. straw, can be collected locally and treated in local,decentralised processing stations where it is stored in compressedcondition before being transported to a central processing plant, e.g. abioethanol plant.

The local treatment thus includes collecting e.g. straw in amounts of10,000-50,000 tons or the like, treating in a straw handling line,pressing etc. as described by the invention, and weighing-in,registration and quality control before storing locally. It is notedthat treatment and compression are here utilised to a total logisticsolution for collecting in the magnitude 0.5-1 m tons of straw or moreto a central bioethanol plant.

Furthermore, acid, base or gases are added as catalysts in the treatmentand at the same time as antimicrobial agents during storage. Hereby isavoided that the biomass is attacked by microorganisms duringstorage—the biomass is quite simply preserved. At the same time it iscleaned, registered and quality controlled via the total pre-treatmentand stored as such in relation to type and quality.

According to a special embodiment, the method according to the inventionis peculiar in that after leaving the reaction chamber, the biomass ismoved directly to a reactor selected among an enzyme reactor, a thermalchemical reactor, a thermal reactor, a chemical reactor, a biologicalreactor, or a different reactor.

According to a special embodiment, the method according to the inventionis peculiar in that after leaving the reaction chamber, the biomass isstored locally and that subsequent processing is performed in a centralplant.

According to a further special embodiment there is indicated a methodfor making fodder, as for example cattle fodder. This is effected byensilation of straw which is treated by the mechanically induced steamexplosion. This enables ensiling the straw, either independently or byan admixing of hay, maize or other crop for ensilation. This improvesthe feed value of straw and mixed ensilations, including increasing thedry matter content, protein content and general digestibility of theensilage.

According to a further special embodiment there is indicated a methodfor treating biomass in the form of wood chips to paper pulp or otherfibre product where the mechanically induced steam explosion constitutesan interjected pre-treatment. This pre-treatment occurs before aconventional thermochemical treatment (KRAFT) in sodium hydroxide (NaOH)and sodium sulphide (Na₂S). This entails that the conventional treatmentcan be effected with less consumption of water, chemicals and energy ina smaller volume, and which therefore is performed in a morecost-effective way.

DESCRIPTION OF THE DRAWING

In the following, the invention will be explained in more detail withreference to the enclosed drawing wherein:

FIG. 1 shows schematically the design of a piston press for use inestablishing mechanical steam explosion in a biomass;

FIG. 2 shows a diagram for illustrating various embodiments of a methodaccording to the invention;

FIG. 3 shows a diagram for illustrating a principle in utilisingmechanical steam explosion as a technique for simultaneous pre-treatmentand feeding of straw into a biogas reactor, alternatively direct orindirect feeding into thermal, chemical, thermochemical or otherbioreactor;

FIG. 4 shows a diagram for illustrating a principle in utilisingmechanical steam explosion as a pre-treatment of straw before abioethanol process and the main principles in the bioethanol process,and wherein lignin is extracted in an enzyme reactor;

FIG. 5 shows a diagram for illustrating a principle in utilisingmechanical steam explosion as a pre-treatment of straw before abioethanol process and the main principles in the bioethanol process,and wherein lignin is removed by a pressing action;

FIG. 6 shows a diagram for illustrating a principle in integration ofmechanically induced steam explosion in a bioethanol process containinga typical thermochemical or other reactor for pre-treatment;

FIG. 7 shows schematically a biogas plant in which is used a pistonpress for establishing mechanical steam explosion in a biomass aspre-treatment of the biomass before introducing into a bioreactor;

FIG. 8 shows a diagram for illustrating a principle in integration ofmechanically induced steam explosion in a method for producing fodder,as for example cattle fodder; and

FIG. 9 shows a diagram for illustrating a principle in integration ofmechanically induced steam explosion in a method for treating biomass inthe form of wood chips into paper pulp or other fibre product before aconventional thermochemical treatment.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

FIG. 1:

Illustrates the technical arrangement and the operation of mechanicalsteam explosion of straw before a biogas process.

In FIG. 1 are used the following reference numbers: 11 is a pistonchamber; 12 is a piston; 13 is a crank; 14 is loose straw; 15 is areaction chamber (pipe) and 16 is compressed straw.

FIG. 2:

Illustrates a flow diagram for utilising the invention for producingbioethanol from straw. The straw 1) is received, torn up and cleaned ina straw handling line before treatment in 2) press and possiblesupplementary hydrolysis before 3) tearing up compressed straw intoloose straw. This loose straw can now be sprayed with or added asuitable mixture of water and enzymes for performing 4) mixing andliquefaction, also called dedicated hydrolysis. Hydrolytic enzymes areadded to water, and this enzyme-water mixture is added to straw suchthat the dry matter content is optimal with regard to hydrolysis as wellas the remaining processes in the total bioethanol production. It isnoted that the invention enables adjusting the dry-matter/water/enzymeratio optimally as the straw is pre-treated in dry condition and is notto be dewatered before hydrolysis, e.g. because the straw has not beenpre-treated by conventional steam-explosion in large amounts of water.The liquefaction or dedicated hydrolysis is effected optimally in thetemperature range 40-80° C., typically 50-55° C. and at pH 4-7,typically pH 5.0-5.5. The duration of the dedicated hydrolysis is 1-100hours, typically 24-72 hours, particularly 48 hours. This dedicatedhydrolysis can be further extended via a membrane enzyme reactor wherethe hydrolysis is extended until the complete decomposition of sugarpolymers into sugar oligomers and monomers. Temperatures and pH whichare optimal to the hydrolysis in a membrane reactor are maintained, andan associated membrane only allows dissolved sugar oligomers andmonomers to pass through the membrane, whereas lignin, unconverted strawand enzymes are retained in enzyme reactor. The enzyme-membrane systemtypically consists of a screening via a vibrating screen, drum screen ormicro-screen for retaining larger particles in enzyme reactor, typicallyparticles between 10 and 200 μm (micrometers), preferably 50-150 μm andtypically under 100 μm. This screened material is now filtered across amembrane, typically an ultrafiltration membrane (UF-membrane) with apore size of 10-100 nm (nanometres), preferably 25-75 nm and typicallyaround 50 nm. Such membranes have a molecular weight cut-off (MWCO) of5-15,000 Dalton and typically around 10,000 Dalton. This membrane allowssugar to pass whereas lignin is retained, constituting a ligninconcentrate. In a preferred configuration, UF-filtration is combinedwith a RO-filtration whereby the dissolved sugars are concentratedbefore fermentation, and where the permeate, the pure water, is recycledto enzyme reactor or before it. The concentrated sugar is supplied to 6)bioreactor for fermentation into bioethanol, subsequent distillationetc. The process around 5) enzyme-membrane reactor can consist ofscreening or UF-membrane only or in combination as well as the membranesystem can include RO-filtration. The most important advantageassociated with the system is that the dissolved sugars—meaning theproduct of the enzyme activity—are continuously removed whereby productinhibition of the enzymes is eliminated. Furthermore, the stay time ofbiomass particles in the enzyme-membrane reactor is disconnected fromthe hydraulic stay time, also contributing to a complete hydrolysis ofthe biomass. Finally, the sugars are concentrated in RO-plant foroptimal concentration of 10-30%, typically around 20%, ensuring anoptimal ethanol concentration during fermentation and distillation.

FIG. 3:

Illustrates a flow diagram for injection of straw into a biogas reactorwherein the straw is torn up and cleaned in a straw handling line beforeactual pre-treatment in the press. The pre-treated and compressed strawcan now be supplied, directly or indirectly, to a biogas reactor, or forthat matter to a different reactor. Here it is utilised that the strawis pre-treated and therefore viscous and easily dissolved in the reactorliquid as well as compressed to high specific density of 0.5-1.5,preferably 0.8-1.2 and typically around 1. It is essential that thecompressed straw has high density as the straw therefore can sink intothe liquid where it is suspended within a short period of time anddistributed within the entire reactor liquid volume. No float layer orother preventing conversion into biogas is thus formed. It is alsoessential that the straw has changed its character and has become veryviscous—i.e. waterabsorbing—as this property allows the straw to besuspended and distributed in the entire reactor liquid volume. Thedirect addition can be effected by connecting the discharge pipe, orextension nozzle, on the press directly to bioreactor while being awarethat compressed straw run in the extension pipe here encounters a liquidwith an overpressure which is proportional to the liquid level in thereactor, e.g. 1 bar or more. However, the compressed straw in theextension pipe is so compressed and is advanced at so great overpressure(up to the mentioned 2000 bars) that the straw, without risking run-backof liquid or leakage of biogas, can be introduced at the bottom ofreactor and therefore under the liquid surface. It is also possible tosupply the straw via another screw system where a long inclining orvertical screw conveyor moves the straw up to a short inclining feedscrew which opens under the liquid surface. Hereby is also avoidedrun-back of liquid and escape of biogas. The straw will also here sinkdown into the reactor liquid and be suspended within a short time. Byshort time is meant between 1 and 120 min, preferably 30-90 min andtypically in less than 1 hour. This is a short period of time in thelight of a typical hydraulic stay time in a biogas reactor of 10-90days. The straw can also be supplied indirectly to bioreactor viaadmixing into another biomass, typically fluid livestock manure, sludge,waster water and the like which is supplied to biogas reactor viapumping. Often a receiving reservoir or receiving tank is provided forliquid biomass in a biogas plant, and the straw can here be added fromthe press, suspended and pumped into bioreactor with the other biomass.If pre-treatment, compression, storage etc. are performed indecentralised collecting stations before transport to the bioenergyplant, the straw will typically be introduced via another screw conveyoror other lock-feeder system.

FIG. 4:

Illustrates a flow diagram for a bioethanol process configurationwherein lignin is removed after pre-treatment and before fermentation,cf. also FIG. 2 (see this). The fermentation and distillation areoptimal, cf. description to FIG. 2, and as the fermentation occurswithout substantial amounts of lignin, the fermentation will result in apure yeast that can be separated from the distillate by centrifugation.The concentrate from the centrifuge constitutes a yeast fraction whereasthe rejected water constitutes a thin liquid fraction with remains ofdissolved sugar, yeast cells, lignin etc. which advantageously can bedegassed in biogas reactor for production of biogas and for conditioningthe liquid before RO-filtration and making of vinasse (K-fertiliser) forfertilising purposes and pure water for recycling. Examples of realistickey figures for production flows are indicated in the Figure. Input is100,000 tons of straw yearly or 12.5 t/h at 8000 operational hours. Thestraw is assumed consisting of 40% cellulose, 30% hemicellulose, 20%lignin and 10% water.

FIG. 5:

Illustrates a flow diagram for a situation in which the biomass containslarger amounts of lignin and where thus a specific lignin extraction isinserted after pre-treatment and before fermentation etc. This ligninextraction has the particular advantage that the pre-treated straw, cf.the invention, is dry and hydroscopic and can therefore be added aliquid which is optimised with regard to the extraction of lignin. In apreferred configuration, organic acids like citric acid, lactic acid,acetic acid and similar organic acids are used for extracting lignin at40-120° C., preferably 60-100° C. and typically 80° C. at final pH of1-6, preferably 2-4 and typically pH 3. It is noted that these acids canbe added before the press, cf. the invention, and if so, only water isadded after the press for lignin extraction. Hereby lignin and partlyhemicellulose and potash salts are extracted whereas pure cellulosefibres are left to further processing. The extraction occurs by adding amixture of water and organic acid to the treated straw after which theliquid after some time undergoes mechanical pressing in one or twosteps. The cellulose fibres continue in the process whereas thelignin-acid mixture is supplied to a biogas process where particularlyhemicellulose and dissolved sugars and the organic acids are convertedinto biogas while lignin passes biogas reactor for subsequentconcentration via UF-membrane. After UF-membrane, K-salts areconcentrated in RO-membrane whereas the permeate, the pure water, isrecycled for renewed extraction. The pure cellulose fibres are suppliedto enzyme membrane reactor, cf. FIG. 2, before fermentation anddistillation, and finally centrifugation for making a pure yeastfraction. In the Figure are mentioned examples of realistic productionfigures and material flows. Input is 100,000 tons of straw yearly or12.5 t/h at 8000 operational hours. The straw is assumed consisting of40% cellulose, 30% hemicellulose, 20% lignin and 10% water.

FIG. 6:

Illustrates in more detail a so-called “whole slurry” processconfiguration where no separation of lignin occurs after pre-treatmentbut where the entire pre-treated biomass is supplied to fermentation anddistillation, and only after distillation it is separated into the maincomponents yeast cells, methane via a biogas process, lignin andvinasse, where vinasse consists of nutrient salts, in particular potash,phosphorus and nitrogen. The configuration is initiated by collectingand a first treatment of straw via 1) a straw handling line where thestraw is torn up to lengths of 1-20 cm, typically 5-10 cm, and iscleaned from contaminants via air-assisted cyclone before a hammer millwhich further reduces the straw length to 0.1-5 cm, typically 1-2 cm,before 2) treatment in mechanical press, cf. the invention. In thatconnection it is possible and probable that the straw is collected,pre-treated, quality controlled, registered, weighed-in and storedlocally in decentralised collecting stations before transport to acentral biogas plant. In the central bioenergy plant, the compressedstraw—in compressed form—is supplied to a 3) thermochemical reactorwhere the straw is added water according to need and subjected to asupplementary hydrolysis via direct injection of steam such that thestraw is exposed to temperatures between 60 and 220° C., typically120-180° C. and particularly 140-60° C., and incubated for a suitabletime, i.e. 1-120 min, typically 10-60 min and particularly 30-40 min.The straw is now ready for 4) enzymatic liquefaction, also calleddedicated hydrolysis, and suitable enzymes are added to water, and thisenzyme-water mixture is added to the straw such that the dry mattercontent is optimal in relation to hydrolysis as well as the remainingprocesses in the total bioethanol production. It is noted that theinvention enables adjusting the dry matter/water/enzyme ratio optimallyas the straw is pre-treated in dry condition and is not to be dewateredbefore hydrolysis. Correspondingly, it is possible to perform asupplementary pre-treatment in the thermochemical reactor with anoptimal ratio between water/dry matter and possible catalysts. Theliquefaction or dedicated hydrolysis is effected optimally in thetemperature range 40-80° C., typically 50-55° C. and at pH 4-7,typically pH 5.0-5.5. The duration of the dedicated hydrolysis is 1-100hours, typically 24-72 hours, particularly 48 hours. Fermentation anddistillation 5 occurs substantially as SSF fermentation (SimultaneousSaccharification and Fermentation), i.e. simultaneous saccharificationand fermentation, and the distillation as vacuum steam distillation, cf.the known principles thereof. A special feature is, however, that thefermentation is extended to 2-14 days, typically 8-12 days andparticularly 10 days against normally 1-3 days for conventionallyoperated plants. This is to achieve maximum specific ethanol productionwhile simultaneously considering the lignin content in the whole slurrysystem. Fermentation occurs at standard pH and temperatures as well asdistillation occurs at standard conditions therefor. During theseparation, 6) separation of yeast cells from the distillate forms partthereof via a new technique adapted to this type of distillatecontaining yeast cells. The distillate is subjected to a “dissolved airflotation”, i.e. injected and dissolved air bubbles which lift up theyeast to the liquid surface where it is conducted away from the liquidand centrifuged. Hereby is achieved a pure yeast substrate which can beused a protein fodder. The residual liquid with a content of dissolvedlignin, residual amounts of sugar, yeast cells and substrate aresupplied to a biofilm reactor for production of biogas. Lignin generallypasses through the biogas reactor whereas residual sugar etc. isconverted to biogas. After biological degassing, the liquid thuscontains a pure lignin fraction and is well suited for settling andultrafiltration for separation of lignin. A pure lignin fraction ishereby produced. At the same time, the UF-filtration enables separationof dissolved nutrient salts from the residual liquid via a finalRO-separation (RO: Reverse Osmosis) and evaporation. The concentratefrom the RO-separation constitutes vinasse while the permeate is purewater which is recycled to steps 3 and 4. Hereby, the production processis complete and thus is produced bioethanol, yeast substrate, methane,lignin and vinasse from the straw.

FIG. 7:

Illustrates a plant including a container 1 that contains a dispensingsilo and a press of the type shown in FIG. 1, two heat treatment screws2, a feeding unit 3, a first conveyor 4, a bioreactor 5, a filling unit6 an a second conveyor 7.

The shown plant operates in that biomass in the form of cut straw,maximum length 40 mm, is filled into the filling unit 6.

The straw is moved on by the second conveyor 7 to the dispensing silo,which is an integrated part of the container 1, and down into the pressin which a briquetting process is performed. After the briquettingprocess, the briquettes are moved via a discharge pipe (also called anextension nozzle) on the reaction chamber of the press to the heattreatment screws 2. The heat treatment screws 2 can be adjusted intemperature and time for passage. The heat treatment screws have acapacity of 750-1200 kg which typically corresponds to one hour ofproduction.

Conveyor 4 moves the briquettes to the feeding unit 3. The feeding unit3 is adapted to introduce the briquette under liquid level in thebioreactor in such a way that gas leakage from the bioreactor 5 will notoccur during the feeding of the briquettes.

Alternatively, the briquettes can be moved by the heat treatment screws2 directly from the piston press into the bottom of the bioreactor 5below liquid level.

FIG. 8:

Illustrates a method for producing fodder, as for example cattle fodder,via ensilation of treated straw. The mechanically induced steamexplosion enables ensiling the straw, either independently or viaadmixing cut grass, maize or other crop for ensilation. This improvesthe feed value of straw and mixed ensilations by i.a. increasing the drymatter content, protein content and general digestibility of theensilage.

FIG. 9:

Illustrates a method for processing biomass in the form of wood chipsinto paper pulp or other fibre product where the mechanically inducedsteam explosion constitutes an interjected pre-treatment before theconventional thermal chemical processing (KRAFT) in sodium hydroxide(NaOH) and sodium sulphide (Na₂S). This entails that the conventionaltreatment can be effected with less consumption of water, chemicals andenergy in a lesser volume, and which therefore overall is performed in amore cost-effective way.

1. A method for processing a biomass containing lignocellulose, saidmethod comprising a water vapour explosion such that cellulose andhemicellulose are made accessible for further processing, whereinprocessed lignocellulose is used for producing biofuels, characterisedin that the method comprises: repeated compressions of the biomass in areciprocating piston press, where loose biomass is continuously fed intoa piston chamber in front of a piston which moves the loose biomass intoa tubular reaction chamber in which the biomass is compressed, and thata pressure of 500 to 3000 bar is applied in the biomass duringcompression whereby the kinetic energy of the piston is deposited in thebiomass in the form of heat, at the same time, the kinetic energy of thepiston is deposited in the straw in the form of heat, said heat and saidrepeated compressions are used for producing a mechanically inducedwater vapour explosion and autohydrolysis under simultaneousdisplacement of compressed biomass through the reaction chamber. 2.Method according to claim 1, characterised in that a pressure between1000 and 2500 bars is applied in the biomass during compression. 3.Method according to claim 1, characterised in that the piston press isadapted such that the temperature in at least a first part of thereaction chamber is within a range from 40° C. to 240° C.
 4. Methodaccording to claim 1, characterised in that the biomass is impregnatedwith gases and/or bases or acids before being introduced in the pistonchamber for pre-processing lignocellulose.
 5. Method according to claim1, characterised in that enzymes and water are added after compression,and that the compressed biomass is subsequently torn up.
 6. Methodaccording to claim 1, characterised in that the compressed biomass istorn up and crumbled after leaving the reaction chamber.
 7. Methodaccording to claim 1, characterised in that after leaving the reactionchamber, the biomass is moved to an enzyme reactor and a subsequentfermentation.
 8. Method according to claim 1, characterised in thatafter compression, the biomass is added to fluid livestock manure orfluid waste water sludge in a biogas plant for a subsequent biogasprocess.
 9. Method according to claim 8, characterised in that thebiomass is added at a level below a liquid surface in a reactor tank ofthe biogas plant.
 10. Method according to claim 1, characterised in thata supplementary hydrolysis is effected after the compressed biomassleaves the reaction chamber.
 11. Method according to claim 1,characterised in that one or more of stone, sand and or other impuritiesare cleaned from the biomass before the biomass is supplied to thepiston chamber.
 12. Method according to claim 1, characterised in thatafter leaving the reaction chamber, the biomass is moved directly to areactor selected among an enzyme reactor, a thermal chemical reactor, athermal reactor, a chemical reactor, a biological reactor, or adifferent reactor.
 13. Method according to claim 1, characterised inthat after leaving the reaction chamber, the biomass is stored locallyand that subsequent processing is performed in a central plant. 14.Method according to claim 1, characterised in that after themechanically induced vapour explosion, the biomass constituted by strawis ensiled for producing forage for cattle forage.
 15. Method accordingto claim 1, characterised in that after the mechanically induced vapourexplosion, the biomass is constituted by wood chips for paper pulp orother fibrous product and is subjected to a conventional thermalchemical processing (KRAFT) in sodium hydroxide (NaOH) and sodiumsulphide (Na₂S).